Daylight harvesting refers to the use of natural daylight to supplement the artificial light in an environment such as an interior space of a building, e.g. an office or other room. The idea can be used to reduce the amount of artificial light needed to illuminate the space and so reduce energy consumption. Nonetheless, in certain environments such as an office workspace a certain standardized or recommend light level may be required, e.g. 500 Lux at desk height. Even in non-regulated environments the end-user may require a certain light level as matter of preference. Hence to conserve energy consumption whilst still meeting the relevant light level requirement, a modern lighting system may comprise a controller which adjusts the artificial light output by one or more electric lighting devices depending on the amount of daylight present.
A closed-loop control system uses feedback of the quantity it is controlling (as opposed to an open-loop system which does not use feedback). In the case of a lighting control system, a photosensor (light sensor) detects the total photometric amount of light from both daylight and electric sources in the space. The sensed level is then used to control the output of one or more electric light sources, to make up any part of the required amount of light that cannot be met using natural light alone.
Generally the output of a light sensor will need to be calibrated. This is done by mapping an observed sensor reading to a known level light in the environment, e.g. by reference to a particular plane of interest such as the workspace plane, thus determining a relationship between the sensor reading and the actual level of light to be measured. Typically this is done by a technician at the commissioning stage using a separate light meter to observe the level of light. Subsequently in operation, the calibrated relationship between sensor reading and light level can then be used to control the output of a lighting device.
For example, in closed-loop control of lighting systems, it may be required that the desired target illumination distribution can be specified. One or more light sensors are used to measure samples of the achieved illumination distribution. In such scenarios the sensor reading corresponding to the specified light level may be used as a calibration point for the light sensor, thus providing a target sensor reading which the lighting system will seek to achieve by adjusting the output of one or more lighting devices. However, for practical reasons, light sensors are often not located on the same plane as the plane where the illumination distribution needs to be rendered and is of most interest. For instance, in office lighting, light sensors are typically located on the ceiling plane, whereas the illumination of interest is that rendered over the workspace plane (e.g. desk level). As such, a calibration method is required to specify the target illuminance values to be attained at the sensors.
With the increased adoption of light emitting diodes (LEDs) in luminaires, it is possible to control the dimming and beam shapes of luminaires flexibly. This has led to a generation of future luminaires wherein multiple beams of flexible light output pattern may be created. For example see US 2010/0264833 (van Endert et al, “Continuous control of LED light beam position and focus based on selection and intensity control”). This technology can be used to design lighting systems which provide dynamic illumination patterns, driven by a desire for greater energy savings and better visual comfort.